Large scale MIMO systems have been widely focused by both the academia and the industry in recent years. Theoretical studies have demonstrated that, both spectral efficiency and energy efficiency of the large-scale MIMO system can be significantly improved by using simple liner algorithms, such as a zero-forcing algorithm, a minimum mean square algorithm and the like. Therefore, the large-scale MIMO system is likely to be adopted as a key technology in next generation communication standards.
However the system performance of the large-scale MIMO system is limited by pilot pollution problem in a scenario of, for example, multi-cell time-division multiplexing. Specifically, since the length of the pilot is limited by a coherence length of a channel the number of orthogonal pilots is limited, and the pilots may be inevitably shared among different cells. In this case, pilot signals transmitted by user equipments in different cells using the same pilot sequence may be received by a base station in a neighboring cell, which, however, cannot effectively distinguish the pilot signals from each other, thereby resulting in interferences to uplink channel estimation at the base station. When performing uplink data detection using the interfered channel estimation, the base station may receive data from user equipments in other cells, besides data transmitted by user equipments in the cell where the base station is located thereby resulting in inter-cell interferences in the uplink. When the base station generates a pre-coding matrix and transmits downlink data using the interfered channel estimation, besides the user equipments in the cell where the base station is located, user equipments in other cells may also receive the data, thereby resulting in inter-cell interferences in the downlink.
Theoretical studies have demonstrated that, although both spectral efficiency and energy efficiency of the large-scale MIMO system can be significantly improved, and the influences of noises and channel estimation error on the system performance are reduced as the number of antennas of the base station is increased, the inter-cell interferences caused by the pilot pollution cannot be eliminated and become one of the factors limiting the performance of the large-scale MIMO system.
In addition, conventional methods for alleviating the pilot pollution are often difficult to be adapted to current technical conditions. Therefore, in actual applications, the pilot pollution is still one of the most serious problems confronted by the large-scale MIMO system.